Medical devices are implanted in human bodies for monitoring physiological conditions, diagnosing diseases, treating diseases, or restoring functions of organs or tissues. As one type of such implantable medical devices, an implantable CRM device monitors a patient's cardiac functions and/or treats cardiac arrhythmias, heart failure, and other cardiac disorders by delivering electrical and/or other therapies to the patient's cardiovascular system including the heart. One example of implantable CRM devices is implantable pacemakers. An implantable pacemaker delivers electrical pulses to the heart to restore cardiac rhythm and coordinate contractions in various cardiac regions, thereby improving hemodynamic performance. Another example of implantable CRM devices is implantable cardioverter/defibrillators (ICDs). ICDs deliver higher energy electrical stimuli to the heart to treat tachyarrhythmias including tachycardia and fibrillation. In addition to pacemakers and ICDs, examples of implantable CRM devices include, but are not limited to, implantable pacemaker/defibrillators that combine the functions of implantable pacemakers and ICDs, implantable drug delivery devices, and any other implantable devices for monitoring, diagnosing, and/or treating cardiac disorders.
Because an implantable CRM device is often intended for long-term use after being implanted in a patient, its size, complexity, and power consumption are inherently constrained. Consequently, a CRM system may include an external device or system and one or more implantable devices. The external device or system provides for a user interface for the CRM system and enables the CRM system to perform functions that the implantable CRM device alone is incapable of performing. Communication between an implantable CRM device and the external device or system is performed by telemetry through an inductive couple or a radio-frequency electromagnetic link. The CRM system may also perform certain functions by coordinated operation of the implantable CRM device and another one or more implantable devices. Such coordinated operation enables the CRM system to perform, for example, monitoring and/or therapeutic functions at multiple locations in a body that are practically difficult to access by using a single implantable device. For this and other reasons, there is a need for an intra-body communication link between two implantable medical devices.